Usually the story of adultery (Madame Bovary, Anna Karenina) ends unhappily. Adultery is so important in the history of the novel because it makes a narrative opportunity out of the state that once brought a novel to its end: marriage. As Tony Tanner puts it in his fascinating book Adultery in the Novel, "the adulterer or adulteress effectively 'renarrativizes' a life that has become devoid of story". Conventionally this creation of a new story brings retribution to the transgressor. Knowing this, the first-time reader of The Forgotten Waltz might wonder if Gina, Enright's protagonist and narrator, is going to be punished.

Gina almost encourages this thought by looking back on her first blundering into infidelity with a cold eye. "Hindsight is a wonderful thing," she says, and all the early episodes in the novel – which otherwise might have been inconsequential – are charged with the knowledge of where they are leading: her affair with Seán, a married man with a child. In the first chapter she recalls seeing him for the first time, at a barbecue in her sister's garden. As elsewhere in the novel when a recollection is peculiarly vivid, she has switched into the present tense. "He is about to turn around, but he does not know this yet. He will look around and see me as I see him and, after this, nothing will happen for many years." It is a false start, but in hindsight it is still a start.

"In hindsight," we say when we mean that we now know better than we did once. Gina first sleeps with Seán at a conference in Switzerland when drunk. Seán "(who is now the love of my life – my goodness, how it betrays him to say this)" performs disappointingly in bed. That parenthesis is characteristic of Enright's narrator. Even as she looks back on the drunken folly of the past she is dizzy with the infatuation of the present. "If you asked me now, of course, I would say I was crazy about him from that first glance." In the novel, however, she does not say this. She recalls being puzzled by him in early encounters, even, after their first night together, "slightly repulsed by him". She is determined not to romanticise her memories.

Hindsight makes the clumsiness of ordering the narrative quite apparent. In the second chapter Gina tells us about her husband, Conor, how she "ended up – this seems a peculiar thing to say – not believing a single thing he did". But then, in the opening line of the next chapter, she corrects herself. "But this was later." "I may be getting things in the wrong order here." "I am getting ahead of myself here." The destination of her narration keeps intruding into her thoughts. Is she headed for an unhappy ending? The lovers sneak a weekend away together and the narrator talks as if it were to finish the affair. "It was the end, I knew that. I think we both knew." Yet Gina says much that is true only to the moment. If we trust to the anchor of hindsight, we should already sense that this is proved false. At the station, before they go their separate ways, she says "No more" and he agrees. Once on the train, she is soon sure that if she does not see him again "I would surely die".

Hindsight imparts a kind of faux wisdom to Gina's reflections. "We talked about Aileen. Of course. We talked about his wife – because that is the thing about stolen love, it is important to know who it is you are stealing from." She sounds as though she is above it all. However, Part II opens with a shift into the present tense and the narrator back in the house of her recently dead mother, with Seán. They are living together. "We are in love," she says, as if this were an only recent discovery. With hindsight Gina can see that her affluence and self-confidence – like that of her generation of young Irish professionals – was entirely fragile. The economy implodes; she has to take a terrible job marketing alcohol. She was a fool, she realises. But not in love. Hindsight should mean distance from what we look back on. The peculiar experience of reading The Forgotten Waltz is the growing recognition that Enright's narrator, for all her self-mockery and her alertness to her own folly, is still caught up in the rapture of her story.

• John Mullan is professor of English at University College London. Join him and Anne Enright on 12 March at 7pm, the Scott Room, the Guardian, 90 York Way, London N1. Tickets £8 – online booking only – at www.guardian.co.uk/books/series/bookclub